Method for manufacturing a printed circuit board element as well as a printed circuit board element

ABSTRACT

A circuit board element and production thereof are disclosed, whereby a noble metal is applied to a structured conductor layer on a circuit board substrate, comprising said conductor layer. The conductor layer is roughened on the surface, preferably after the structuring thereof and the noble metal applied as a layer, essentially on all of the structured roughened conductor layer, whereupon the noble metal layer surface is given a corresponding roughness.

The invention relates to a method for manufacturing a printed circuitboard element, wherein starting from a printed circuit board substratewith at least one conductor layer, preferably two conductor layers,these or at least one conductor layer, respectively, is structured andnoble metal is applied thereon.

The invention further relates to a printed circuit board element with atleast one, preferably two structured conductor layer(s) applied on asubstrate, and with a noble metal on the or at least one conductorlayer, respectively.

The term “printed circuit board element” is to be understood asone-sided or double-sided printed circuit board or also as a multilayerprinted circuit board, mounted or not yet mounted with components,wherein it is primarily essential that a substrate, usually from anepoxy resin layer, is present with at least one metallic, electricallyconductive layer, normally made of copper, provided thereon.Hereinafter, the metallic layer is designated as “conductor layer”. Saidconductor layer may be an outer layer or also, in the case of amultilayer, an inner layer.

It has already been proposed to locally provide on sites where electriccomponents are to be mounted a noble metal layer, especially made ofsilver, on the copper conductor layer. This particularly happens whenPTF components (PTF—polymer thick film) is mounted by means of aprinting process, wherein here especially PTF resistors are used. Thenoble metal layer between the copper of the conductor layer and the PTFpaste of the components enhances the stability of the circuit, whichcircuit may be negatively affected by, e.g., moisture; the noble metallayer forms a “blocking layer” insofar as it helps to avoid corrosion ofcopper on account of moisture, e.g. when the PTF paste hardens. On theother hand, it improves the electric contact between the copper of theconductor layer and the PTF paste.

This, however, involves the drawback that the noble metal layer can onlybe applied on locally very limited sites, namely exactly on sites to beimprinted with the PTF paste; providing the noble metal layer on thewhole copper conductor layer, would, however, highly negatively affectthe adherence of layers to be applied thereon. For example, in the caseof a pressing to the multilayer, the necessary interlaminar adherence toa printed circuit board structure provided thereon would no longer bepresent, and the result would be a delamination when mounting on such aprinted circuit board element, and thus a total defect of the printedcircuit board element. If the conductor layer is an outer layer, i.e. noepoxy-resin/conductor layer structure is to be mounted, it is, as arule, necessary for mounting components to create a solder stop mask onthe outer layer, and in the case of a noble metal layer such a solderstop mask would adhere only badly. For these reasons, as mentionedabove, the noble metal can only be applied on locally very limitedsites, and the remaining copper conductor layer surface, when furtherproceeding the printed circuit board element, is covered with a separateadherence promoting layer to promote either the mounting of a solderstop mask or pressing to the multilayer.

For the only partial application of the noble metal on locally limitedareas of the conductor layer, however, separate process steps arerequired, i.e. applying of a masking layer and removing thereof;nevertheless, spalling of the materials present thereon occurs over andover again in the region of the noble metal surface, and thus results indefects.

It is now the aim of the invention to provide a method for manufacturinga printed circuit board element and a printed circuit board element,respectively, of the initially defined kind, wherein efforts and costsfor the process steps of masking and demasking during manufacture can beavoided, so that the manufacture is essentially simplified, and wherein,nevertheless, an excellent adherence of the individual layers to eachother is rendered possible, no separate (additional) adherence promotinglayer on the conductor layer being necessary.

To achieve this aim, the invention provides a method and a printedcircuit board element, respectively, as defined in the independentclaims.

Advantageous embodiments and further developments are indicated in thedependent claims.

With the inventive technique, the noble metal layer itself forms anadherence promoting layer on which subsequent layers, such as a furtherprinted circuit board structure or also a solder stop mask, can beapplied with good adherence thereto. Moreover, the material of PTFcomponents produced by imprinting, especially PTF resistors, but alsosolder material, or, in the case of an adhesive fastening, adhesivematerial of prefabricated components excellently adheres to the noblemetal layer. This good adherence of the different materials is achievedby using a roughened surface of the noble metal layer instead of asmooth one. Said surface roughness is created already in the conductorlayer, usually made of copper, present under the noble metal layer,wherein the surface roughness is provided in an order of magnitude of atleast that of the thickness of the noble metal layer, preferably beinglarger by one order of magnitude. Thus, said surface roughness of theconductor layer is also maintained when subsequently applying the noblemetal layer. Particularly, the surface roughness of the conductor layeris in the range of between 0.05 μm and 5 μm, especially 0.3 and 3 μm,preferably 0.5 μm and 1 μm, and the preferred thickness of the noblemetal layer is between 0.02 μm and 1 μm, preferably between 0.02 μm and0.5 μm. The surface of the conductor layer may be roughened, e.g. bychemical etching, mechanical processing or by electroplating. The noblemetal layer is then applied on said roughened conductor layer, e.g.electroless or by electroplating, by cathodic evaporation or sputtering.It is advantageous to use silver, gold, palladium, nickel or acombination of the individual ones of these metals or all of the metals,as noble metal. After having applied said comparably thin noble metallayer on the whole surface of the conductor layer (instead of applyingonly locally on small partial sections), wherein the surface roughnessis maintained, electrical components, such as, in particular, PTFresistors, etc, can be applied by imprinting, and a pressing to amultilayer may follow or also a solder stop mask may be mounted in orderto allow for the attachment of prefabricated electric components in anautomatic soldering process.

An exemplary production according to the inventive technique providesfor the following basic steps:

a) structuring the copper conductor layer (usually by etching in aphotolithographic process)

b) roughening the surface of the conductor layer

c) applying the noble metal layer

d) applying the components (e.g., PTF resistors)

e) pressing to a multilayer.

Compared thereto, in the conventional standard process the followingprincipal steps are necessary:

a) structuring the conductor layer

b′) applying a separate adherence promoting agent on the conductor layer(usually by means of an oxidation process and by the aid of organicconstituents)

c′) applying and structuring a photoresist layer

c″) partial applying a noble metal layer

c′″) removing the photoresist layer

d) applying the components

e) pressing to the multilayer

As can be directly learned from this comparison, cost-intensive stepsare rendered unnecessary by the inventive technique, if proceedingaccording to the invention; furthermore, the properties of the printedcircuit board elements are improved.

Hereinafter, the invention is further explained by way of preferredexemplary embodiments, yet without being restricted thereto, and by thedrawings. In detail, the drawings show in:

FIG. 1 a schematic top view on a printed circuit board element withlocal noble metal layers on a copper layer according to the prior art;

FIG. 2 a schematic section through a part of said printed circuit boardelement on a larger scale according to line II-II of FIG. 1;

FIG. 3 a schematic top view on a printed circuit board element accordingto the invention in correspondence with the illustration of FIG. 1; and

FIG. 4 a cross section through said printed circuit board elementsimilar to the illustration of FIG. 2, wherein additionally a printedcircuit board structure, which is to be pressed on, having a copperouter layer as well as a bottom conductor layer is illustrated.

In FIG. 1 a part of the printed circuit board element 1 is schematicallyshown in top view, the layered structure of the printed circuit board 1furthermore being apparent from FIG. 2 by way of example.

In FIG. 1 individual materials of the printed circuit board element 1,as they are visible on the upper side of the printed circuit boardelement 1, are illustrated by hatched lines, dotted lines ordouble-hatched lines for a better differentiation, wherein thesingle-hatched lines illustrate epoxy resin substrate 2 (cf. also FIG.2), on which a structured metal layer 3, especially made of copper, ispresent, which layer is illustrated by dotted lines for a betterdifferentiation. Hereinafter, said metal layer 3 is referred to asconductor layer 3 for the sake of simplicity, and by its structuring (bymeans of photolithographic techniques) these areas on the upper sidehave become visible, i.e. have been uncovered, in which the epoxy resinsubstrate 2 lying thereunder, hereinafter generally referred to asprinted circuit board substrate 2 or shortly substrate 2, can be seenfrom the upper side.

Furthermore, double-hatched lines refer to electric components 4 whichare components, in particular resistors, e.g. imprinted by means of PTFtechnique (PTF Polymer Thick Film). In the region of the ends of saidcomponents 4, in particular resistors, connecting or contact surfaces,respectively, are provided, which are formed by the corresponding areas5′ (cf. FIG. 2) of conductor layer 3 as well as of further noble metallayers 6 locally applied thereon. In FIG. 1 said noble metal layers 6are illustrated as free surfaces without any hatches or the like, in thecross section illustrated in FIG. 2, however, with hatches.

Noble metal layers 6 consists, in conventional manner, of, e.g. a thinlocal silver layer, and they form a blocking or stabilizing layer; whencomponents 4 are mounted and fixed (hardened) said layer hindersmoisture from reaching conductor layer 3 and thus prevents corrosion ofsaid conductor layer 3 in the contact area. Furthermore, said noblemetal layer 6 also improves the electric contact between component 4 andconductor layer 3.

This known technique involves the drawback that the noble metal layer 6can be applied only locally, in very limited areas, namely in the areasof contact surfaces 5. If such noble metal coating was applied on thewhole conductor layer 3, a subsequent coating of the printed circuitboard element 1, as illustrated in FIGS. 1 and 2, e.g. with a furtherprinted circuit board structure or also with a solder stop mask, wouldnot adhere at all or only very poorly, and this would result in arelatively quick detachment of the applied layer(s).

On the other hand, applying the noble metal layers 6 only locallyrequires relatively great effort during manufacture, as separated stepsare necessary for masking the upper side of the printed circuit boardelement 1 as well as for removing said masking after having appliednoble metal layers 6. Moreover, practice shown that, when further layersare applied on layers 6, local spalling of said applied layers occurs inthe area of noble metal layers 6, e.g., when printed circuit boardelement 1, as illustrated in FIGS. 1 and 2, is pressed to a multilayer(cf. also FIG. 4), and, in particular, spalling of the materials ofcomponents 4 (PTF resistors) off the noble metal layers 6 has beenobserved.

The inventive technique allows for a complete coating of conductor layer13 on substrate 12 with noble metal, as can be seen in FIG. 3, and as aresult, conductor layer 13 is fully or substantially fully covered witha noble metal layer 16. The above-mentioned masking and demaskingprocesses, as required in the known technique, are thereby renderedunnecessary. Nevertheless, excellent adherence of the layers appliedthereon is rendered possible, e.g. the adherence of a substrate 3′ of aprinted circuit board structure 1′, which structure has an outerconductor layer 3′, when pressed to a multilayer, cf. FIG. 4 in whichsaid pressing (simultaneously with a heating) is schematicallyillustrated by arrow 7. Instead of such a printed circuit boardstructure 1′, as shown in FIG. 4, e.g. also a solder stop mask can bemounted onto the upper side of the printed circuit board element 11according to FIGS. 3 and 4. Also in this case good adherence isobtained.

To obtain this good adherence, conductor layer 14 is roughened on itssurface before applying noble metal layer 16, wherein surface roughness8 shown in FIG. 4 is illustrated quite schematically by a wavy line. Thecomparatively thin noble metal layer 16 also follows said surfaceroughness 8 of conductor layer 13, so that its upper side also shows acorresponding roughness 8′. This surface roughness 8′ then guaranteesthe mentioned good adherence of a layer applied thereon, e.g. of anepoxy resin substrate layer 2′ or a solder stop mask.

Printed circuit board substrates of the conventional type, such as, e.g.FR 4 substrate, can be the basis for the present printed circuit boardelement 11, wherein, as mentioned above, an epoxy resin substrate 2 anda copper layer 13 applied thereon are present. If possible, substrate 2can also be provided with a conductor layer (copper layer) 13′ on thebottom side, as illustrated in FIG. 4, and said conductor layer 13′ canalso be correspondingly roughened and provided with a noble metal layer.The mounted component 4 preferably is a polymer thick-film component, inparticular a PTF resistor, which is attached in a printing process. Sucha PTF resistor consists, e.g., of phenolic resin, in which, depending onthe resistance value, carbon particles are included in the requiredconcentration.

Noble metal layer 16 may consist of silver, as already mentioned, butalso of gold, palladium, platinum or nickel or similar noble metals.Depending on the materials used, also corresponding roughening andapplication techniques, respectively, are employed. These techniques areknown per se and therefore they are only briefly mentioned hereinafter.

In the case of a noble metal layer 16 made of silver, the basis for thecoating preferably constitutes a simple exchange reaction, wherein thecopper of conductor layer 3 is replaced by silver of noble metal layer16 according to the relation

Ag_(aq) ^(++½)Cu_(s)→Ag_(s)+½Cu_(aq) ²⁺  (1)

Said exchange reaction happens between copper and silver on account ofthe potential difference, and a very compact silver layer on the copperlayer is obtained. The result are silver layers 16, of a thickness inthe range of between 0.1 μm and 0.25 μm (generally preferably between0.02 μm and 0.5 μm).

In the case of a palladium coating, hydrogen and an activator containingpalladium is applied, whereby an autocatalytic reaction is initiated:

Pd°±H₂→Pd°+2H_(ads)  (2)

Pd⁺⁺+2H_(ads)→Pd°+2H  (3)

$\begin{matrix}{{2\; {{Red}.}}->{{\frac{\left\lbrack {Pd}^{\; \circ} \right\rbrack}{{{pH} < 3},5}H_{2}} + {CO}_{2}}} & (4)\end{matrix}$2Red.+Pd⁺⁺→Pd°⁺+2H+CO₂  (5)

At a pH of <3.5 decomposition of the reduction agent starts, andreactions occur in the diffusion layer only. The released hydrogenguarantees the continuance of the reaction.

Comparable chemical coating techniques can be used for nickel and gold.

Other per se conventional techniques can, however, also be used forapplying the noble metal layer 16 on the conductor layer 3, such as,e.g., attaching of the noble metal on the conductor layer 13 byelectroplating, attaching of the noble metal by cathodic evaporation orapplication by sputtering. The noble metal layer, as mentioned above,advantageously has a thickness of between 0.02 μm and 1 μm.

After of before being conventionally photolithographically structured,the surface of conductor layer 13 can be roughened by means of variousper se conventional techniques. For example, roughening by a mechanicalprocess is conceivable, as well as by electroplating or by ionic orchemical etching, respectively, wherein an etching medium, e.g. on thebasis of hydrogen peroxide/sulfuric acid, is used. The surface roughnessis provided in an order of magnitude of between 0.05 μm and 5 μm,preferably 0.3 μm and 3 μm, in particular 0.5 μm and 1 μm, and partlyneedle-like surface structures are obtained. (As mentioned above, thewave-like illustration of FIG. 4 is to be viewed only veryschematically, it is, however, also provided to make clear that surfaceroughness 8 and 8′, respectively, is greater, i.e. is provided in alarger dimensions compared to the thickness of noble metal layer 16.)

1. A printed circuit board element with at least one structuredconductor layer (13) on a substrate (12), which substrate has aroughened surface (8), and with metal (16) on the conductor layer (13),characterised in that a surface-rough noble metal layer (16) is appliedon the whole roughened conductor layer (13) and serves as contactpromoting and stabilizing layer, on the one hand, and as adherencepromoting layer, on the other hand.
 2. A printed circuit board elementaccording to claim 1, characterised in that a further printed circuitboard structure (1′) is provided on the surface-rough noble layer (16),forming a multilayer configuration.
 3. A printed circuit board elementaccording to claim 1, characterised in that a solder stop mask ismounted on to the surface-rough noble metal layer (16).
 4. A printedcircuit board element according to claim 1, characterised in that atleast one electric component (4) is mounted on the surface-rough noblemetal layer (16).
 5. A printed circuit board element according to claim4, characterised in that the electric component (4) is a resistor, e.g.a PTF resistor.
 6. A printed circuit board element according to claim 1,characterised in that the conductor layer (13) and the noble metal layer(16), respectively, has a surface roughness (8; 8′) of between 0.05 μmand 5 μm, e.g. 0.3 μm and 3 μm, preferably 0.5 μm and 1 μm.
 7. A printedcircuit board element according to claim 1, characterised in that thenoble metal layer (16) has a thickness of between 0.02 μm and 1 μm,preferably 0.02 μm and 0.5 μm.
 8. A printed circuit board elementaccording to claim 1, characterised in that the noble metal layer (16)has at least one metal selected from the group consisting of silver,gold, palladium and platinum.
 9. A printed circuit board elementaccording to claim 1, characterised in that the substrate (12) has twostructured conductor layers (13, 13′), wherein on at least one layer anoble metal layer (16) is applied.